6-Amino-spiro[penam-2,4'-piperidine]-3-carboxylic acid derivatives having the formula ##STR1## wherein R is a methyl, phenyl or benzyl radical, Z.sub.1 is a hydrogen atom and Z.sub.2 is a radical selected from those known from penicillin chemistry and is preferably a 2-phenylacetyl, 2-amino-2-phenylacetyl, 5-methyl-3-phenyl-4-isoxazolecarbonyl or 2,6-dimethoxybenzoyl radical, or Z.sub.1 and Z.sub.2 together represent a bivalent radical Z.sub.3 and preferably a (hexahydro-1H-azepin-1-yl)methylene radical, as well as the pharmaceutically acceptable non-toxic salts thereof and process for preparing the same. These compounds have valuable antibacterial properties and are useful as therapeutic agents in the treatment of infectious diseases caused by Gram-positive and Gram-negative bacteria.